As concerns about clean energy such as solar power for coping with exhaust of fossil resources and environmental pollution increase, a solar cell generating an electromotive force using sunlight has been the subject of recent research.
Solar cells generate an electromotive force from diffusion of minority carriers, which are excited by sunlight, in P-N (positive-negative) junction layer. Single crystalline silicon, polycrystalline silicon, amorphous silicon or compound semiconductor may be used for the solar cells.
Although solar cells using single crystalline silicon or polycrystalline silicon have a relatively high energy-converting efficiency, solar cells using single crystalline silicon or polycrystalline silicon have a relatively high material cost and a relatively complicated fabrication process. Accordingly, a thin film type solar cell using amorphous silicon or compound semiconductor on a cheap substrate such as glass or plastic has been widely researched and developed. Specifically, a thin film type solar cell has advantages in a large-sized substrate and a flexible substrate so that a flexible large-sized solar cell can be produced.
FIG. 1 is a cross-sectional view of an amorphous silicon thin film type solar cell according to the related art. In FIG. 1, a first electrode 12, a semiconductor layer 13 and a second electrode 14 are sequentially formed on a substrate 11. The transparent substrate 11 includes glass or plastic. The first electrode 12 includes a transparent conductive oxide (TCO) material for transmission of incident light from the transparent substrate 11. The semiconductor layer 13 includes amorphous silicon (a-Si:H). In addition, the semiconductor layer 13 includes a p-type semiconductor layer 13a, an intrinsic semiconductor layer 13b and an n-type semiconductor layer 13c sequentially on the first electrode 12, which form a PIN (positive-intrinsic-negative) junction layer. The intrinsic semiconductor layer 13b, which may be referred to as an active layer, functions as a light absorption layer for improving efficiency of the thin film type solar cell. The second electrode 14 includes a TCO material or a metallic material such as aluminum (Al), copper (Cu) and silver (Ag).
When sunlight is irradiated onto the transparent substrate 11, minority carriers diffusing across the PIN junction layer of the semiconductor layer 13 generate a voltage difference between the first electrode 12 and the second electrode 14, thereby generating an electromotive force.